This invention relates to a circularly polarized (CP) microstrip antenna, more particularly to a circularly polarized microstrip antenna with a broad CP bandwidth. The invented antenna is useful, for example, in automobile-mounted apparatus for receiving transmissions from earth satellites.
Since the orientation of an automobile-mounted antenna with respect to a transmitting antenna on a satellite is unfixed, the automobile-mounted antenna must be able to receive transmitted radio waves regardless of the direction of their electric field vector, which is to say that the antenna must be circularly polarized. CP microstrip antennas can be found in the prior art. Japanese Patent Application Kokai Publication 281704/1986, for example, discloses a CP microstrip antenna having a disk-shaped antenna element with diametrically opposed notches.
The circular polarization characteristic of this prior-art microstrip antenna is satisfactory, however, in only an extremely narrow frequency band. Moreover, the impedance bandwidth of this antenna is extremely narrow: a slight deviation from the optimum frequency causes impedance mismatching, leading to reflection at the interface between the antenna element and its feed line.
The impedance bandwidth problem is also encountered in rectangular "patch" microstrip antennas. Improvement by addition of a rectangular parasitic director element in front of the driven antenna element has been described in, for example, "Influence of Director Size upon a Microstrip Quadratic Patch Bandwidth" by G. Dubost, J. Rocquencourt, and G. Bonnet in the IEEE 1987 International Symposium Digest, Antennas and Propagation, pp. 940-943, 1987. Placement of an analogous disk-shaped director in front of the circularly polarized microstrip antenna described above also improves its impedance bandwidth, but not its CP bandwidth. Tests have in fact shown that such a director has a strongly adverse effect on circular polarization.